1. Field of the Invention
This invention relates to liquid organosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction. More particularly, this invention relates to low viscosity curable organosiloxane compositions that exhibit physical properties in the cured form that previously could only be achieved using relatively high viscosity organosiloxane compositions.
2. Background Information
Many types of ingredients for modifying the properties of cured material prepared from organosiloxane compositions are described in the literature. For example, methods for increasing the tensile and tear strengths of organosiloxane elastomers include increasing the molecular weight of the curable polyorganosiloxane, adding reinforcing fillers such as silica and solid resinous organosiloxane copolymers containing groups that react with the curable polyorganosiloxane and/or the curing agent during curing of the composition. Increasing the molecular weight of the curable polyorganosiloxane and adding a reinforcing filler will increase the viscosity of the curable organosiloxane composition.
For some end use applications of liquid curable organosiloxane compositions such as encapsulating and mold making it is desirable to have a viscosity below about 100 Pa.s to ensure that the composition fills all of the crevices in the mold master or the device to be encapsulated and can be easily freed of air bubbles generated when the ingredients of the compositions are blended. Many compositions that cure by a platinum catalyzed hydrosilation reaction do not have sufficient stability to be stored as one-part materials for any length of time. The organohydrogensiloxane and platinum catalyst are typically packaged in separate containers and are combined shortly before it is desired to cure the composition.
In addition to a low viscosity prior to curing, the cured elastomer used to prepare molds must have sufficient integrity, strength and flexibility to withstand repeated molding cycles during which the mold is substantially deformed and sometimes turned inside out to remove the molded object. Depending upon the type of material being molded the mold can be subjected to temperatures as high as 150.degree. C. and pressures up to 80 p.s.i. (552 kPa.) generated during the formation of polyurethane foams.
The use of resinous polyorganosiloxanes containing reactive groups to decrease the viscosity of organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction is described in U.S. Pat. No. 4,882,398, issued on Nov. 21, 1989. The copolymers are solid at room temperature and contain vinyl or other ethylenically unsaturated hydrocarbon radicals that react with the organohydrogen-siloxane(s) present in the curable composition. In addition, the viscosity-reducing effect is observed only with compositions containing liquid polydiorganosiloxanes exhibiting viscosities greater than 12 Pa.s at 25.degree. C.
Organosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction and include liquid polydiorganosiloxanes containing alkenyl radicals and liquid copolymers containing monofunctional R.sub.3 SiO.sub.1/2 or M units and difunctional R.sub.2 SiO or D units where R represents a hydrocarbon radical that reacts during curing of the composition are described in U.S. Pat. No. 5,082,886 and U.S. Pat. No. 3,957,713. Combinations of MQ resins with gum type polydiorganosiloxanes for use as pressure sensitive adhesives are described in U.S. Pat. No. 4,521,471.
One objective of this invention is to provide organosiloxane compositions with viscosities below about 100 Pa.s at 25.degree. C. The compositions cure by a platinum-catalyzed hydrosilation reaction to yield strong, tough elastomers that in the past have required high viscosity curable compositions that do not have the flow properties desired for mold making and other applications.